Nonlinear control strategies and vehicle traction control

Control of vehicle traction is of utmost importance in providing safety and obtaining desired vehicle motion in longitudinal and lateral vehicle control. Vehicle traction control systems can be designed to satisfy various objectives of a single vehicle system or a platoon of vehicles in an automated highway system, which include assuring ride quality and passenger comfort. Other objectives are aimed at providing desirable longitudinal and lateral motion of the vehicles.; Vehicle traction force directly depends on the friction coefficient between road and tire, which in turn depends on the wheel slip as well as road conditions. Wheel slip is the difference between the wheel velocity and the vehicle velocity normalized by the wheel velocity or the vehicle velocity. From the control point of view, we may influence traction force by varying the wheel slip. The dynamics for the traction system are highly nonlinear and uncertain, which motivates the use of sliding mode control strategy to follow a target slip. The sliding mode controller designed for vehicle traction control is made adaptive to reduce the control discontinuity around the switching surface of the sliding mode. A sliding mode based estimation scheme is developed and used to estimate the road tire conditions for maximum acceleration and maximum deceleration. The main problem with sliding mode control is the high frequency chattering across the switching surface. A boundary layer is introduced around the switching surface and a methodology is developed for designing various functions to be used within the boundary layer for chattering reduction and error convergence in continuous time and discrete time sliding mode control of a class of nonlinear systems. This methodology is used to design appropriate functions which are used in vehicle traction controllers. Different objectives of traction control give different target slips to be followed. Simulation study, in this thesis, shows that longitudinal controllers, which do not take traction into account explicitly (termed as tractionless or passive controllers) cannot handle severe external disturbances well. On the other hand, longitudinal traction controllers (termed as active controllers) give satisfactory results with the same disturbances. Simulations show how some of the vehicle performance objectives are met by using traction controllers.